1. Field of the Invention
The present invention relates to an apparatus for controlling the rotating speed of a plurality of DC motors and, more particularly, to an apparatus of the type in which each DC motor is equipped with an encoder for detecting the rotating speed.
2. Description of the Prior Art
Conventionally, for example, in copying machines, chains and clutches and the like are coupled to an AC motor, and a drum and a fixing roller and the like are driven by a sole motor due to the mechanical interlocking. However, recently, in many cases, each of the optical system driving unit, drum driving unit, fixing roller driving unit, etc. independently uses a motor (particularly, a DC motor equipped with an encoder).
It is a well-known technology that an encoder is provided for a DC motor and a speed command value is compared with an output of the encoder and the DC motor is rotated at a constant speed in response to the speed command.
Such a technology is disclosed in the article: "A MICROPROCESSOR SPEED CONTROL SYSTEM", IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL INSTRUMENTATION, VOL. IECI 24, No. 3, August 1977, pages 241 to 247. The circuit is disclosed in FIG. 1 in page 241.
A fundamental circuit the PLL (phase locked loop) control which is equivalent to the system disclosed in FIG. 1 is shown. As shown in this drawing, in the PLL control, by making the phase of the feedback signal from the encoder coincide with the phase of the reference frequency signal corresponding to the command speed of the motor, the motor can be rotated at a constant instructed speed.
However, in this example, one controller is provided for one motor and nothing is mentioned with regard to the control for a plurality of motors.
In OA (office automation) equipment such as the above-mentioned copying machine, printer or the like, there is a tendency such that the equipment is driven by a plurality of independent motors. However, it is very difficult to control a plurality of motors by a sole controller. This is because there is a difference between the frequency of the reference pulse and the frequency of the feedback pulse, so that it is very difficult to match the phases in control of a plurality of motors having different rotating speeds.
Therefore, when a plurality of motors are controlled, the reference frequency and circuit constant are determined for every motor and a dedicated controller is used for each motor.
Consequently, the controllers of only the number of motors are needed, resulting in a large circuit scale.